1. Field of the Invention
The invention relates to an arrangement for controlling the viscosity of a fluid having a constant output pump and a measuring capillary tube through which said output is forced, in which the differential pressure of said fluid between the beginning and the end of said capillary tube is used as a control signal so as to influence the viscosity of that fluid.
2. Description of the Prior Art
The prior art is shown in U.S. Pat. No. 2,771,770, which patent discloses a capillary tube viscosity meter, the output signal of which is used so as to automatically control viscosity. Said viscosity meter can readily be installed in a pipe line so that the viscosity of the fluid flowing in this line will be measured under the temperature actually present in the line, there being no need for extracting fluids out of said line for measuring purposes. In said known capillary tube viscosity meter a fluid, the viscosity of which has to be measured, is forced through a measuring capillary tube, the pressure difference of the fluid at the beginning and the end of the measuring capillary tube being measured. The pump and the capillary tube are integrated in such a way, that they can be arranged within the line, consequently they are surrounded by fluid flowing in the line, while the suction inlet of the pump and the outlet end of the capillary tube are in direct communication with the interior of the line.
Such viscosity meter can be easily installed in a line which supplies the heated fuel to a fuel burner installation or an injection internal combustion engine; in such systems not only proper operation but also the power consumption of the high pressure pump depend strongly on the viscosity of the fuel, so the fuel is heated in order to reach the wanted low viscosity. The known viscosity meter is adapted to continually measure the viscosity at the temperature actually present in the fluid line or to control the viscosity because whether in the pump nor in the capillary tube a decrease in temperature of the fluid will occur.
The above, however, has the drawback that the accuracy of the viscosity measurement also depends on the static pressure at the point where the fluid discharges from the measuring capillary tube. Until now the pressure variations in the static pressure were restricted, but recently fuel systems are used having automatic means such as self cleaning filters, which introduce relatively high, partly periodic, pulse-shaped pressure variations in the fuel system. In the prior art viscosity control system these pulsations, which are superimposed on the proper control signal, are transferred onto the means which influence the viscosity of the fluid. Generally, in such control systems the first means is a differential pressure transmitter. This differential pressure transmitter is influenced by the pulsations in a non-linear way. As a result, errors are introduced in the processing of said control signal and under serious conditions the differential pressure transmitter could be forced completely out of its normal linear control range.